The present invention relates to optical filters, and particularly to filters which can be used for separating interleaved channels from a WDM signal or for interleaving groups of channels.
Wavelength division multiplexing (WDM) is a technique widely used in optical communications systems to allow different wavelengths to be carried over a common fiber (or fiber optic waveguide). The most commonly used wavelength band for fiber optic transmission is centered at 1550 nm, because of the low absorption and the commercial availability of erbium doped fiber amplifiers which are effective for this band.
Wavelength division multiplexing can separate this band into multiple channels, typically 32 or 64 multiple discrete channels, through a technique referred to as dense channel wavelength division multiplexing (DWDM). This is used to increase long-haul telecommunication capacity over existing fiber optic transmission lines.
Techniques and devices are required, however, for multiplexing and de-multiplexing the different discrete carrier wavelengths. Multiplexing and de-multiplexing will typically take place not only at the source and destination of the data, but also at intermediate routing locations, particularly if a network provides per-channel routing capability and per-channel add and drop functions. Thus, each routing device requires the multiplexing and de-multiplexing capability.
Various types of optical multiplexer are well known for the combination or separation of optical signals in wavelength division multiplexed systems. Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tuneable filters.
Regardless of the type of multiplexer and de-multiplexer to be employed, as the channel separation is reduced in order to increase the system capacity, the multiplexer filtering characteristics need to be improved to ensure separation of channels without cross talk between adjacent channels.
Channel interleaving is one technique which has been used to enable two groups of channels to be processed separately, with the channels in each group having greater separation. This reduces the required optical performance of the optical components used within the processing equipment for each group of channels. This invention is particularly directed to an optical filter which enables two groups of interleaved channels to be separated or combined.
According to the invention, there is provided an optical filter comprising a plurality of stacked optical cavities each having substantially the same thickness, the filter having a first frequency response for transmission through the filter comprising a first comb response and a second frequency response for reflection from the filter and comprising a second comb response, the peaks of the first comb response lying between the peaks of the second comb response.
The multiple cavity arrangement in this filter enables the peaks in the reflection response and the peaks in the transmission response to be broadened (with respect to a single cavity response), so that the filter can be used for transmitting one group of channels and reflecting another group of channels at interleaved positions. The cavities are preferably formed from silicon wafers, so that existing techniques can be employed to obtain specific cavity thicknesses with sufficient accuracy and uniformity.
The thickness of the cavities is selected as a function of the channel spacing, and determines the separation between the peaks of the two comb responses. The spacing between cavities is selected as function of the wavelength band over which the filter is to be used.
The thickness of each cavity may therefore be selected such that the spacing between peaks of each comb response corresponds to double the channel spacing in a WDM optical communications system in which the filter is to be used.
The spacing between adjacent cavities may be equal to one quarter of a wavelength within a band of wavelengths of channels of a WDM optical communications system in which the filter is to be used.
Preferably, adjacent cavities are separated by an air spacing. The large refractive index difference between silicon and air enables a small number of cavities to achieve the required filter response. For example, only three cavities can achieve the required pass band profile for DWDM optical communications systems.
The thickness of the silicon wafer of each cavity may be between 180 and 210 microns, and this is appropriate for use with a channel separation of 0.8 nm which is the standard 100 GHz grid. More preferably in this example, the thickness of the silicon wafer of each cavity is between 194.5 and 195.0 microns.
A support layer may be provided between the cavities for defining the air spacing, the support layer comprising an oxide layer.
This layer may define a spacing between adjacent cavities of 0.36-0.41 microns. This range corresponds to one quarter of the wavelength of signals in the C-band.
The invention also provides a method of manufacturing an optical filter, comprising:
preparing a silicon wafer having a predetermined thickness;
depositing an oxide layer over the silicon wafer;
patterning the oxide layer to define a plurality of pattern portions;
dividing the wafer into sections, each section comprising a pattern portion;
stacking a plurality of the sections to define a multiple cavity filter, the wafer sections each defining a cavity and the pattern portions defining a spacing between adjacent cavities.
This method enables a single silicon wafer to be used to form all cavities of the filter, thereby ensuring uniform thickness and enabling precise thickness control using conventional silicon wafer processing techniques. For example, double sided polishing may be used, and with feedback control based on optical thickness measurement, for example infrared interferometric sensing.
The invention provides a low cost process which can be accurately controlled.
The spacing between cavities is preferably determined by an oxide layer, which can also be deposited and patterned to the requited accuracy using conventional semiconductor device processing technology.
The pattern portions may each comprise a central opening around which one or more support portions of predetermined thickness are provided. The opening then defines the air cavity in the assembled device, and the support portion provides the required cavity separation.
The silicon wafer is preferably prepared to have a thickness between 180 and 210 microns. The oxide layer may be deposited with a thickness of 0.32-0.44 microns, preferably 0.37-0.39 microns.
The stacked sections are preferably diffusion bonded.
The invention is also directed to the use of an optical filter of the invention for interleaving or de-interleaving two groups of channels of a WDM optical signal. The invention also provides an optical communications system using a filter of the invention for interleaving or de-interleaving two groups of channels of a WDM optical signal.